Bone resection apparatus and method for knee surgery

ABSTRACT

A bone resection apparatus and its method of use for knee surgery are presented. In one aspect of the invention, the apparatus includes a spacer and a tibial cut guide. The spacer has a first surface engageable with the cut distal femoral bone and a second surface opposite the first surface engageable with the uncut proximal tibial bone in joint articulating relationship. The spacer further includes a cut guide support. The tibial cut guide has a cutter guide defining a cut plane and a support engaging element engageable with the cut guide support. The cut guide support and support engaging element cooperate to position the cut plane in relative to the first surface.

FIELD OF THE INVENTION

The invention relates to a bone resection apparatus and method for kneesurgery. In particular, the invention relates to an apparatus forspacing and resecting a tibial bone during knee replacement surgery.

BACKGROUND

Degenerative and traumatic damage to the articular cartilage of the kneejoint can result in pain and restricted motion. Knee replacement surgeryis frequently utilized to alleviate the pain and restore joint function.An incision is made into the knee joint to expose the bones comprisingthe joint. Cutting guides are used to guide the removal of the articularsurfaces that are to be replaced. Artificial joint components arepositioned to replace the resected bone ends in order to establish thedesired alignment and mechanics of the joint. In a total kneereplacement, all of the articulating compartments of the joint arerepaired with prosthetic components. However, often only one compartmentof the knee joint, typically the medial compartment, is impaired. Thus,in a unicondylar knee replacement, only the damaged compartment isrepaired with prosthetic bearing components.

FIGS. 1-3 illustrate several aspects of implant orientation. FIG. 1illustrates various axes of the lower limb in the frontal plane. Axescan be defined for each segment of the lower limb. For example, thefemur 1 has an anatomic axis 2 coinciding generally with itsintramedullary canal. It also has a mechanical axis 4, or load axis,running from the center of the femoral head to the center of the knee.The angle 6 between these two axes 2, 4 in the frontal plane varieswithin the patient population but is on the order of 4-9′. The two axes2, 4 are approximately superimposed in the sagittal plane (FIG. 2).Likewise, the tibia 3 has a mechanical axis 5 coinciding generally withits intramedullary canal. The mechanical axis 5 of the tibia runs fromthe center of the knee to the center of the ankle. The transverse axis,or joint line 8, about which the knee flexes, is parallel to a linethrough the medial and lateral femoral condyles and parallel to thetibial plateau. Typically, the distal femur and proximal tibia areresected to be parallel to the joint line 8, and thus perpendicular tothe mechanical axes 4, 5 as indicated at 10 and 12. The intersection ofthe femoral and tibial mechanical axes 4, 5 may subtend a small anglerelative to one another. However, the angle is small such that the axes4,5 are approximately collinear ay be treated as collinear for mostpurposes.

FIG. 2 illustrates the knee joint from the side or sagittal view andvarious bone cuts that may be made to align implant components. Thedistal femoral cut 10 is typically made perpendicular to the femoralaxes 2, 4 in the sagittal plane. The proximal tibial resection 12 istypically cut to match the natural posterior slope, or rotation, 16 ofthe proximal tibia relative to the mechanical axes 4, 5. The amount ofposterior slope 16 relative to a reference line 18 perpendicular to themechanical axes 4, 5 varies in the patient population but is on theorder of T. The distance between the distal femoral cut 10 and proximaltibial cut 12 along the mechanical axes 4, 5 is the extension gap. Othercuts may be made depending on the components that are to be implanted.These include an anterior femoral cut 20, anterior femoral chamfer cut22, posterior femoral chamfer cut 24, and posterior femoral cut 26. Thepatella 7 may also be cut 28 to allow for replacement of the patellararticular surface. In a unicondylar knee replacement, only the medial orlateral side of the knee joint is resurfaced. Furthermore, thetrochlear, or patellar bearing, surface of the femur is typically leftintact in a unicondylar procedure. Unicondylar implant designs vary, buttypically only the distal femoral cut 10, posterior femoral chamfer cut24, and posterior femoral cut 26 are needed to accommodate theunicondylar femoral implant.

FIG. 3 depicts six aspects of component positioning relative to acoordinate system in which the x-axis 30 corresponds approximately tothe joint line 8, the z-axis 34 corresponds approximately to themechanical axes 4 and 5, and the y-axis 32 is normal to the other two.Position along each of thee axes is depicted by arrows. Position alongthe x, y, and z axes determines the medial/lateral (dx) 36,anterior/posterior (dy) 38, and proximal/distal (dz) 40 positioning ofcomponents respectively. Rotation about each of these axes is alsodepicted by arrows. Rotation about the z-axis (rz) 42 correspondsanatomically to external rotation of the femoral component, rotationabout the x-axis (rx) 44 corresponds to extension plane rotation, androtation about the y-axis (ry) 46 corresponds to varus/valgus rotation.

SUMMARY

The present invention provides a bone resection apparatus and method forknee surgery.

In one aspect of the invention, the apparatus includes a spacer and atibial cut guide. The spacer has a first surface engageable with the cutdistal femoral bone and a second surface opposite the first surfaceengageable with the uncut proximal tibial bone in joint articulatingrelationship. The spacer further includes a cut guide support. Thetibial out guide has a cutter guide defining a cut plane and a supportengaging element engageable with the cut guide support. The cut guidesupport and support engaging element cooperate to position the cut planein predetermined posterior slope angular relationship to the firstsurface.

In another aspect of the invention, the apparatus includes an implant, aspacer, and a tibial cut guide. The implant has a femoral componentthickness, a tibial component thickness, and an overall implantthickness which is the sum of the femoral component thickness, thetibial component thickness, and an additional joint laxity distancecorresponding to a desired amount of joint laxity. The spacer has a bodyincluding a planar seating surface, an arcuate condylar surface archingaway from the seating surface generally in the shape of an anatomicfemoral condyle, and an elongated support having a longitudinal axis.The spacer has a spacer thickness normal to the planar seating surface.The tibial cut guide has a body with a front surface, a back surface,and an aperture through the body from the front surface to the backsurface having a longitudinal axis. The aperture is engageable with thesupport for linear translation parallel to the aperture longitudinalaxis. A cutler guide slot extends through the body from the frontsurface to the back surface. The cutter guide slot defines a cut planeoriented parallel to the aperture longitudinal axis. The cut plane isspaced a predetermined distance from the aperture such that with theaperture engaged with the support the cut plane is spaced from thearcuate condylar surface a tibial resection distance.

In another aspect of the invention, a method of performing knee surgeryincludes: resecting a portion of the distal femoral bone; inserting aspacer into the knee joint to abut the cut surface of the femoral bone,the spacer having an arcuate condylar portion facing away from the cutsurface; abutting the arcuate condylar portion with the proximal tibialsurface; mounting a tibial cut guide on the spacer to position a cutplane at a predetermined posterior slope angle and depth relative to thecut surface of the femoral bone; and guiding a cutter in the cut planewith the cut guide to form a planar surface on the tibia.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples of the present invention will be discussed withreference to the appended drawings. These drawings depict onlyillustrative examples of the invention and are not to be consideredlimiting of its scope.

FIG. 1 is a front elevation view of a tibia and a femur showing axes ofthe knee joint;

FIG. 2 is a side section view of a knee joint showing typical bone cutsused in replacing the joint surfaces;

FIG. 3 is a perspective view of knee joint showing aspects of componentpositioning;

FIG. 4 is a side elevation view of a knee joint with a distal condyleresected according to the present invention;

FIG. 5 is a side elevation view of the knee joint of FIG. 4 showing aspacer according to the present invention and the knee being articulatedinto extension;

FIG. 6 is a side elevation view of the spacer of FIG. 5;

FIG. 7 is a front elevation view of the spacer of FIG. 5;

FIGS. 8-9 are front elevation views of an alternative anatomicallyshaped spacers;

FIG. 10 is a side elevation view of the knee joint of FIG. 4 showing acut guide in use with the spacer of FIG. 5;

FIG. 11 is a front elevation view of the knee joint, cut guide, andspacer of FIG. 10;

FIG. 12 is a side elevation view of a unicondylar knee implant;

FIG. 13 is a side elevation view of the cut guide and spacer of FIG. 10;

FIG. 14 is a side elevation view of the cut guide of FIG. 10 shown inuse with a thicker spacer;

FIG. 15 is a front elevation view of a tibia and femur showing a varusknee joint;

FIG. 16 is a front elevation view of a tibia and femur showing a normalknee joint; and

FIG. 17 is a front elevation view of a tibia and femur showing a valgusknee joint.

DESCRIPTION OF THE ILLUSTRATIVE EXAMPLES

Embodiments of the present invention include a spacer sized and shapedto temporarily replace a resected femoral condyle to permit noinialarticulation of the knee joint intraoperatively. The spacer includes aseating portion for seating on the cut surface of the bone and acondylar portion arching away from the seating portion generally in theshape of an anatomic femoral condyle. The seating portion may engage thebone surface in a simple frictional engagement. The seating portion mayinclude a roughened surface, barbs, spikes, pins, and/or other fixationenhancement features to fix it in position on the bone surface. Separatefasteners such as pins, screws, clips, clamps, and/or other fastenersmay also be used to fix the spacer in position. Separate fasteners mayengage surfaces, grooves, slots, holes and/or other features of thespacer to hold it in position. For example, the seating portion mayinclude a flat surface frictionally engageable with the cut surface ofthe femur. The condylar portion of the spacer may be curved in thesagittal plane to match the anterior/posterior curvature of the femoralcondyle. The condylar portion may be curved in the frontal plane tomatch the medial/lateral curvature of the femoral condyle. The spacermay be provided in a variety of sizes and curvatures to permit selectionof a spacer that best matches the patient's anatomy. For example, thespacer may be provided in a variety of sizes to permit selection of aspacer that blends smoothly with the resected femur anteriorly,posteriorly, medially, and laterally to permit a gapless, orapproximately gapless, fit to the bone and smooth anatomic articulationof the knee joint. The spacer may have a condylar thickness parallel tothe mechanical axis of the femur. The spacer may be provided indifferent condylar thicknesses to permit selection of a spacer thicknessto replace the same thickness of bone that was resected to restorepre-surgical kinematics. The spacer thickness may also be chosen to belarger or smaller than the bone removed to correct a varus or valgusabnomiality of the pre-surgical knee.

The spacer may include a cut guide support. The support may include ahole, slot, groove, rail, beam, stem, handle, and/or other structure forsupporting a cut guide relative to the spacer. The support may form apredetermined angle relative to the seating portion to position the cutguide at a predetermined angle relative to the seating portion in thesagittal plane. For example the support may incorporate a predeterminedposterior slope angle relative to the seating portion. The spacer may beprovided in different versions with different posterior slope angles.For example, the posterior slope angle may vary from zero degrees to tendegrees. For example, versions may be provided with three degrees, fivedegrees, and seven degrees of posterior slope. The cut guide support mayengage the cut guide to permit varus/valgus rotation of the cut guiderelative to the seating portion in the frontal plane. For example, thecut guide support may have a circular cross section to permitvarus/valgus rotation of the cut guide about the support axis. Thesupport may engage the cut guide to restrain the cut guide from rotatingrelative to the support. For example, the support may have anon-circular cross section to rotationally constrain the cut guide withrespect to varus/valgus rotation. For example, the support may have a“T”-shaped, “D”-shaped, “I”-shaped, and/or other non-circularly shapedcross section. For example, the support may include a “T”-shaped sternprojecting away from the support at an angle to the seating portion.

A tibial cut guide for use with the spacer may include a body having asupport engaging element and a cutter guide. The support engagingelement may be positionable at various positions along the support toallow the distance between the cut guide and the bone to be adjusted.The support engaging element may include a hole, slot, groove, rail,beam, stem, handle, and/or other structure for engaging the support. Forexample, where the support includes a stem extending away from thespacer, the support engaging element may include a complimentary shapedaperture through the cut guide body. The cut guide may be slidable alongthe support to position the cut guide adjacent to the tibia. The cutterguide may include a planar surface, slot, rail, and/or other suitablecutter guide defining a cut plane and able to guide a saw blade, burr,mill, and/or other suitable cutter within the cut plane. For example,the cutter guide may include a slot defining a cut plane. The slot mayreceive a saw blade and constrain the saw blade to motion within the cutplane to produce a planar surface on the tibia. The cut plane defined bythe slot may be oriented parallel to the support engaging element in thesagittal plane such that adjustment of the cut guide along the supportdoes not change the cut plane location relative to the tibia.

The cut guide may further include a fixation element to accommodateattaching the cut guide to the tibia. The fixation element may include aroughened surface, barbs, spikes, pins, and/or other fixationenhancement features to fix the cut guide in position on the tibia.Separate fasteners such as pins, screws, clips, clamps, and/or otherfasteners may also be used to fix the cut guide in position. Separatefasteners may engage the bone and surfaces, grooves, slots, holes and/orother features of the cut guide to hold it in position. For example, thecut guide may include fixation holes for receiving pins to attach thecut guide to the tibia. The fixation element may be oriented parallel tothe cut plane and support engaging element so that the cut guide may beremoved and repositioned easily on the bone. For example, where thefixation element includes holes for receiving pins, the holes may beparallel to the cut plane and support engaging element so that the cutguide can be slipped off of the pins and support, the knee repositioned,e.g. from extension to flexion, and the cut guide slipped back onto thepins. For example, this may be advantageous where it is desirable to setthe cut guide with the knee in extension but where it may be safer ormore convenient to cut the tibia with the knee in flexion.

FIG. 4 illustrates a knee joint 50 defined by a distal femur 52 and aproximal tibia 54. The invention will be illustrated in use to perform aunicondylar knee surgery in which a single compartment of the knee isreplaced including a portion of one femoral condyle and a portion of theproximal tibia 54. However, it is contemplated that instruments andmethods according to the present invention ay also be used in a totalknee replacement in which both the medial and lateral portions of theknee are resurfaced. In FIG. 4 a distal portion 56 of the medial femoralcondyle has been resected using conventional techniques such as guidinga saw blade to resect the portion 56 while the knee is in flexion. FIGS.5-7 illustrate a spacer 60 according to the present inventionpositionable on the cut surface 58 of the distal femur 52 to temporarilyreplace the resected portion 56. The spacer permits the knee joint 50 tobe articulated between flexion and extension to check the jointkinematics and adjust the soft tissue balancing and limb alignment. Ifadjustments are necessary, the surgeon can recut the femur, release softtissues, and/or try different thickness spacers 60 until the desiredkinematics are achieved. The spacer 60 includes a generally arcuate body62 having a planar ng portion 64 for seating on the cut surface 58 ofthe bone and an arcuate condylar portion 66 arching away from theseating portion generally in the shape of an anatomic femoral condyle.In the illustrative spacer, the seating portion 64 engages the bonesurface in a simple frictional engagement. The condylar portion 66 iscurved both in the sagittal plane (FIG. 6) to match theanterior/posterior curvature of the femoral condyle and the frontalplane (FIG. 7) to match the medial/lateral curvature of the femoralcondyle. The spacer 60 is provided in a variety of sizes and curvaturesto permit selection of a spacer that best matches the patient's anatomy.As best seen in FIGS. 5 and 11, the spacer 60 blends with the distalfemur 52 to provide an approximately gapless fit to the bone and smootharticulation of the joint. The spacer 60 has a condylar thickness 68parallel to the mechanical axis 4 of the femur. The spacer 60 isprovided in different condylar thicknesses 68 to permit selection of aspacer thickness 68 to replace the same thickness of bone 56 that wasresected to resto p surgical kinematics or a different thickness 68 tocorrect a varus or valgus abnormality of the presurgical knee. FIG. 7illustrates a universal spacer 60 that has a generic projected frontalshape allowing it to be used on both medial and lateral sides of theknee joint 50. FIGS. 8 and 9 illustrate optional spacers 60 a and 60 bhaving anatomical frontal shapes that may provide a better fit to themedial and lateral sides of the knee 50. The universal spacer 60 of FIG.7 reduces the required inventory of spacers while the spacers 60 a and60 b of FIGS. 8 and 9 improve the medial/lateral fit of the spacer tothe bone.

The spacer 60 includes a cut guide support 70 formed as a “T”-shapedstem projecting anteriorly from the body 62. The support forms apredetermined posterior slope angle 72 with the seating portion 64 toposition the cut guide 100 to cut the proximal tibia 54 at a desiredposterior slope angle. The spacer 60 is provided in different versionswith different posterior slope angles that vary from zero degrees to tendegrees. Preferably, a plurality of spacers 60 is provided with a fewrepresentative angles including three degrees, five degrees, and sevendegrees of posterior slope.

The cut guide 100 includes a body 102 having a front surface 104, a backsurface 106, a top 108, a bottom 110, and side walls 112, 114. A“T”-shaped aperture 116 extends through the body 102 from the frontsurface 104 to the back surface 106. The aperture 116 engages thesupport 70 and constrains the cut guide 100 to linear translationaladjustment along the support 70 so that the cut guide 100 can be slippedonto the support 70 and slid along the support 70 until it abuts theproximal tibia 54. The engagement of the aperture 116 and support 70maintain a fixed posterior slope angular relationship between the cutguide 100 and the seating portion 64 and a fixed varus/valgus angularrelationship between the cut guide 100 and the seating portion 64.Typically, it is desirable to have the tibial and femoral cut planesoriented at the same varus/valgus angle relative to the knee to avoidfemoral component edge loading during articulation of the reconstructedjoint. The illustrative resection apparatus maintains parallelvarus/valgus rotation by way of the rotationally keyed support 70 andaperture 116. The parallelism of the slot 118 and the cut distal femoralsurface 58 of the illustrative resection apparatus can be seen in FIG.11.

However, there may be surgical situations in which it is desirable tovary the varus/valgus angular relationship of the femoral and tibial cutplanes. By providing an optional keyless support 70 and aperture 116 thetibial cut plane 120 may be angled in varus/valgus rotation relative tothe seating surface 64 and thus relative to the cut distal femoralsurface 58. For example, the support 70 and aperture 116 may be circularto allow varus/valgus rotation about the support 70 axis.

A saw slot 118 extends through the body 102 from the front surface tothe back surface 106. The saw slot 118 defines a cut plane 120 (FIG. 10)and guides a saw blade 119 within the cut plane to form a planar surfaceon the proximal tibia 54. The cut plane 120 is oriented parallel to theaperture 116 such that adjustment of the cut guide 100 along the support70 does not change the cut plane location on the proximal tibia 54.Fixation holes 122 are formed through the body 102 from the frontsurface 104 to the back surface 106 to receive fixation pins 123 tomaintain the cut guide's 100 position on the proximal tibia 54. Thefixation holes 122 are oriented parallel to the cut plane 120 andaperture 116 so that the cut guide 100 may be removed from the support70 and fixation pins 123 and then replaced on the fixation pins 123during surgery if desired by the surgeon. For example, afterestablishing the cut guide 100 position, the surgeon may desire toremove the spacer 60 and/or reposition the knee before making the tibialcut. With the fixation pin holes 122 parallel to the support aperture116, the cut guide 100 may easily be slid off of the pins 123 andsupport 70 and subsequently replaced on the pins 123 in the sameposition.

In knee replacement surgery, an incision is made to expose a portion ofthe knee. The illustrative unicondylar implants and instruments aresuitable for a minimally invasive approach to the knee in which theincision is minimized to reduce trauma to the patient and speed recoveryfrom the surgery. In particular, the simple, streamlined design of thespacers 60 and cut guides 100 permits them to be positioned throughnarrow incisions. The support 70 also is convenient as a handle formanipulating the spacers into and out of the incision. In anillustrative procedure, the knee 50 is flexed to approximately ninetydegrees of flexion (FIG. 4) and a portion 56 of the distal femur 52 isresected. The resection is oriented so that the cut surface 58 of thedistal femur 52 is perpendicular to the mechanical axis of the 4 of thefemur 1. A spacer 60 is chosen that fits the cut surface 58 of thedistal femur 52 and is positioned with the support 70 facing anteriorly(FIG. 5). The knee 50 is articulated through its range of motion withthe tibia bearing on the condylar surface 66 of the spacer 60 while thesurgeon assesses the limb alignment and soft tissue balancing. Ifadjustments are necessary, the surgeon may recut the femur, release softtissues, and/or try a different thickness of spacer. When the surgeon issatisfied with the knee 50 kinematics, he returns the knee 50 to fullextension and slides a cut guide 100 onto the support 70 (FIG. 10) untilthe back surface 106 of the cut guide 100 abuts the proximal tibia 54.The angle of the support 70 relative to the seating portion 64establishes the posterior slope of the proximal tibia 54 relative to themechanical axis of the femur. With the knee 50 in full extension, thesaw slot 118 defines a cut plane 120 corresponding to the desiredposterior slope angle. The cut guide 100 is fixed to the tibia byinserting pins through the fixation holes 122 and into the proximaltibia 54. With the cut guide 100 fixed to the proximal tibia 54, a sawblade 119 is guided through the saw slot 118 to resect a portion 55 ofthe proximal tibia in the cut plane 120. Optionally, the cut guide 100can be removed from the fixation pins before the tibial resectionperformed to permit the spacer 60 to be removed and/or the knee to berepositioned into a more convenient or safer position for the cut. Thecut guide 100 can then be replaced over the pins to restore the desiredorientation of the cut guide 100 and the resection performed. Additionalfemoral bone cuts are made as necessary to finish the femur to receivethe femoral implant.

An illustrative implant 200 is shown in FIG. 12. The implant 200includes a femoral component 202 having a femoral component thickness204 and a tibial component 206 having a tibial component thickness 208.The components 202, 206 are shown spaced apart a small amountcorresponding to a desired joint laxity 210. A smaller spacing resultsin a tighter knee and a larger spacing results in a looser knee. Thetotal of the femoral component thickness 204, the tibial componentthickness 208, and the joint laxity 210 is represented by an overallimplant thickness 212.

As seen in FIGS. 13 and 14, the thickness 68 of the spacer 60 and therelationship of the saw slot 118 to the support aperture 116 determinesthe depth of the tibial cut 124. The cut depth 124 is shown beingmeasured at approximately one-half the anterior/posterior distance ofthe tibial plateau as indicated by the centerline 126. The distance 128from the seating portion 64 to the cut plane 120 corresponds to theoverall implant thickness 212. For surgery on a knee with normalpre-surgical limb alignment (FIG. 16), the spacer thickness 68 is chosento correspond to the femoral component thickness 204 plus the desiredjoint laxity 210. This is evaluated during the articulation andadjustment discussed relative to FIG. 5. The resulting tibial cut depth124 will then correspond to the tibial component thickness 208 and thepost-surgical limb alignment will match the pre-surgical alignment.However, for correction of a varus knee (FIG. 15), the spacer thickness68 is chosen to correct the limb alignment and will be different fromthe femoral component thickness 204. For unicondylar replacement of themedial compartment, increasing the bone spacing on the medial side 220of the knee 50 will correct the limb alignment. Thus, inserting athicker spacer 60 during the articulation and adjustment step (FIG. 5)of the operation will correct the alignment. During the tibial resectionstep of the surgery, the increased spacer 60 thickness 68 decreases thetibial cut depth 124 as seen in FIG. 14. When the implant components areinserted into the prepared knee, the tibial component will be positionedhigher and thus the joint line 8 on the medial side of the knee 50 israised and the varus deformity is corrected. For a valgus deformity(FIG. 17), a thinner spacer can be used to increase the tibial cut depth124 and lower the joint line 8 on the medial side of the knee 50.

Although examples of a bone resection apparatus and its use have beendescribed and illustrated in detail, it is to be understood that thesame is intended by way of illustration and example only and is not tobe taken by way of limitation. The invention has been illustrated in thecontext of a spacer and resection guide for spacing and resecting themedial side of the tibial plateau during unicondylar knee replacementsurgery. However, the bone resection apparatus may be configured inother shapes and for use at other locations within a patient's body.Accordingly, variations in and modifications to the bone resectionapparatus and its use will be apparent to those of ordinary skill in theart, and the following claims are intended to cover all suchmodifications and equivalents.

1. A system for guiding a bone cut comprising: at least one spacerhaving a first surface configured to engage a surface of a cut distalfemoral bone, a second surface opposite the first surface configured toengage an uncut proximal tibial bone and to reestablish a jointarticulating relationship between the cut femoral bone and the uncuttibial bone; a tibial cut guide defining a cut plane and a supportengaging element; and a cut guide support engageable with the at leastone spacer and the tibial cut guide to position the cut plane at apredetermined posterior slope angle relative to the first surface. 2.The system of claim 1, wherein the surface is planar.
 3. The system ofclaim 1, wherein the second surface comprises an arcuate condylarsurface generally in the shape of an anatomic femoral condyle.
 4. Thesystem of claim 1, wherein the second surface is curved in a sagittalplane to correspond to an anterio/posterior curvature of an anatomicfemoral condyle.
 5. The system of claim 4, wherein the second surface iscurved in a frontal plane to correspond to a medial/lateral curvature ofan anatomic femoral condyle.
 6. The system of claim 4, wherein the atleast one spacer comprises a plurality of spacers having a convexlycurved second surface, each one of the second surfaces having adifferent curvatures to permit selection of a spacer that best matches apatient's anatomy or that best corrects an anatomical defect in thepatient.
 7. The system of claim 3, wherein the at least one spacer has acondylar thickness, the at least one spacer comprising a plurality ofspacers each having a different condylar thickness.
 8. The system ofclaim 1, wherein the cut guide support comprises an elongated memberhaving a longitudinal axis and in an assembled configuration the cutguide support projects outwardly from the at least one spacer.
 9. Thesystem of claim 1, wherein in an assembled configuration with the atleast one spacer a longitudinal axis of the cut guide support forms apredetermined tibial posterior slope angle relative to the firstsurface.
 10. The system of claim 1, wherein the at least one spacercomprises a plurality of spacers and wherein in an assembledconfiguration with the cut guide support each one of the plurality ofspacers has a different posterior slope angle between zero and tendegrees.
 11. The system of claim 1, wherein the cut support comprises anon-circular cross section and the tibial cut guide comprises a anaperture having a non-circular cross section configured to receive thesupport, and wherein in an assembled configuration the aperture and cutguide support permit linear translation of the cut guide relative to theat least one spacer and maintain a fixed posterior slope angle betweenthe cut guide and the at least one spacer.
 12. The system of claim 1,wherein the cut guide support comprises a circular cross section and thetibial cut guide comprises an aperature having a circular cross sectionconfigured to receive the cut guide support, and wherein in an assembledconfiguration the aperture and the cut guide support permit lineartranslation of the cut guide relative to the at least one spacer,varus/valgus rotation of the cut guide relative to the at least onespacer, and maintain a fixed posterior slope angle between the cut guideand the at least one spacer.
 13. The system of claim 1, wherein the cutguide includes a saw slot defining the cut plane, and wherein in anassembled configuration the cut plane is oriented parallel to thelongitudinal axis of the cut guide support.
 14. The system of claim 1,wherein the cut guide includes at least one fixation hole having alongitudinal axis, the at least one fixation hole being oriented suchthat in an assembled configuration the longitudinal axis of the fixationhole is parallel to the longitudinal axis of the cut guide support. 15.A system comprising: an implant system including a femoral componenthaving a femoral component thickness and a tibial component having atibial component thickness, the implant system having an overall implantthickness which is the sum of the femoral component thickness, thetibial component thickness; and an additional joint laxity distancecorresponding to a desired amount of joint laxity; at least one spacerhaving a body including a planar seating surface, an arcuate condylarsurface arching away from the seating surface generally in the shape ofan anatomic femoral condyle, and an elongated support having alongitudinal axis, the spacer having a spacer thickness normal to theplanar seating surface; and a tibial cut guide including an aperturehaving a longitudinal axis, the aperture configured to engage the cutguide support for linear translation of the cut guide parallel to theaperture longitudinal axis, a cutter guide slot defining a cut planeoriented parallel to the aperture longitudinal axis, the cut plane beingspaced a predetermined distance from the aperture such that in anassembled configuration the cut plane is spaced from the arcuatecondylar surface by a tibial resection distance.
 16. The apparatus ofclaim 15, wherein the at least one spacer comprises a plurality ofspacers, each one of the plurality of spacers having a different spacerthickness, wherein in an assembled configuration each one of thepluarlaity of spacers establishes a different tibial resection distance.17. The apparatus of claim 15, wherein in an assembled configuration thecut plane is spaced from the planar seating surface a distance equal tothe overall implant thickness.
 18. A method of performing knee surgerycomprising: resecting a portion of a distal femoral bone; inserting aspacer into the knee joint to abut a cut surface of the distal femoralbone, the spacer having an arcuate condylar portion facing away from thecut surface; abutting the arcuate condylar portion with a surface of aproximal tibial bone: mounting a tibial cut guide on the spacer toposition a cut plane at a predetermined posterior slope angle and at adepth relative to the cut surface of the distal femoral bone; andguiding a cutter in the cut plane to form a planar surface on theproximal tibial bone.
 19. The method of claim 18, further comprising,before the step of mounting the tibial cut guide, the steps of:articulating the knee joint between flexion and extension such that thesurface of the proximal tibial bone articulates with the arcuatecondylar portion of the spacer to evaluate knee kinematics; and placingthe knee joint in extension.
 20. The method of claim 18, furthercomprising, after guiding a cutter, mounting a femoral implant componenton the resected distal femoral bone and mounting a tibial implantcomponent on the proximal tibial bone, wherein inserting a spacercomprises inserting a spacer having a thickness corresponding to thefemoral implant component thickness plus an additional thicknesscorresponding to a desired joint laxity such that the cut guide guidesthe cutter to remove a portion of proximal tibial bone equal inthickness to the tibial implant component thickness.